The present invention relates to a power source control system for data terminal equipment, which is connected to a communication line.
In recent years the processing performance of personal computers has made rapid progress and its communication facility has also markedly advanced. In consequence, the personal computers have materialized efficient data communications therebetween through a dedicated line or telephone network, and have come into wide use as an intelligent data terminal in a public data transmission service network, computer system, LAN, or similar other applications. In any case, there is a form of communication, which unilaterally transmits information to the data terminal or acquires information held in the data terminal. In this instance, there is a form of communication which permits the so-called automatic reception without the necessity of the attendance of an operator to a called data terminal. The automatic reception herein mentioned is the same as employed in conventional telex or facsimile communication system, whereby the power supply to the data terminal is turned ON upon receiving an incoming call and is turned OFF after completion of the communication message. This improves safety against accidents during the absence of an operator, for example, at night, and offers advantages of a more efficient utilization of equipment and less power consumption.
From such a point of view, an attempt has been made at the automatic reception also when the personal computer is employed as a data terminal. However, the automatic reception of prior art is intended to apply for an equipment which enables a communication immediately after the application of power to the equipment. In case of the personal computer, it is needed to load an operating system (OS) and communication software thereon after the application of power, and there must be a certain amount of time before the transmission of communication message. Consequently, the prior art could not be applied, as it is, to the personal computer.
That is, since functioning the personal computer as a data terminal involves the execution of a communication protocol, it is necessary that a program having such a function (a communication program) run on the personal computer. At the initial stage of a personal computer where the power source of the personal computer is OFF, the communication program exists on a magnetic disc or like nonvolatile memory, along with an operating system (OS). When the power source is turned ON, the operating system is loaded on a main memory through the operation of an initial program loader (IPL) and then, by the operation of the operating system, the communication program is loaded and executed. The program loading time until the running of the program varies with the property of the program but usually ranges from dozens of seconds to one minute.
Because of the above-noted characteristic of the personal computer, a nonanswering time would be made to the calling party for dozens of seconds to one minute in the prior art which turns ON the power to the personal computer immediately after the detection of the incoming call. In a case where the called party is always specified as in the case of communication using a dedicated line, such a phenomenon could be avoided by preparing the communication program of the calling party in anticipation of this nonanswering time. In a public telephone network or public data network, however, the time interval between the arrival of an incoming call and its answering operation is generally limited and if no answer is given within the limited period of time, the line is disconnected by assuming that the called data terminal is not in operation. Accordingly, in the prior art there is a possibility that no communication line is established although the called data terminal makes preparations for the incoming call.
Incidentally, according to the "Rules concerning technical standards relating to a contract for the use of a public communication line (telephone)" of NTT (NIPPON TELEGRAPH AND TELEPHONE CORPORATION), it is prescribed that the automatic originating function is to open a DC circuit within 60 seconds after sending a select signal when the called party does not answer.
As described above, when the personal computer is used as a data terminal, the prior art which turns ON the power to the personal computer simply upon the detection of an incoming call (a ringing) does not satisfy the above-noted rules, resulting in the problem that no communication line is established.